1. Field of the Invention
The present invention relates to a method for forming a photoresist pattern using a chemically amplified resist.
2. Related Arts
Large scale integrated circuits (hereinafter referred to as "LSI") are integrated higher and higher every year, so that a size reduction of the circuits under the design rule thereof is required more. Further, a light source used in a photolithography process has been shifted from an i-line (wavelength 365 nm) to a KrF (wavelength 248 nm) excimer laser and then to an ArF (wavelength 193 nm) excimer laser.
Generally, a chemically amplified resist to be able to photosensitive by using an excimer laser comprises a resin as a main component and a photosensitizer which generates a strong acid by exposure of the excimer laser light. The acid generated by exposure cuts a side chain of the resin and, at the same time, acts the photosensitizer which is not exposed yet to chain-react for generating more acid. Cutting the side chain of the resin and generating the acid, which are induced by the heat treatment (PEB: post-exposure baking) in succession, make a soluble region and an insoluble region against a developer.
As mentioned above, it is necessary for the chemically amplified resist, in consideration of its nature, to generate the strong acid in the case of light exposure. However, its acid is apt to be neutralized by moisture, ammonia or the like in the air to be deactivated during the time before shifting from the exposure to the PEB process (hereafter, required time before the PEB process after the exposure is referred to as "delay") as shown in FIG. 2(a). As a result, the content of acid is low in the surface of the resist. Accordingly, as for a positive resist, a surface thereof becomes slightly soluble in the developer, so that T-shape patterns are formed on its surface as shown in FIG. 2(b). As for a negative resist, a surface thereof is apt to be soluble in the developer.
FIGS. 2(a) and 2(b) are figures served for explaining problems in the conventional technique. In FIGS. 2(a) and 2(b), reference numeral 21 represents a substrate to be etched, 22 represents a chemically amplified resist film and 23 represents a resist pattern.
In order to prevent this deactivation of the acid, a process control with as short delay as possible, an environmental cleaning by means of a chemical filter, an overcoat of a strong acid aqueous solution and the like have been considered. Further, Unexamined Japanese Patent Publication No. Hei 7(1995)-295228 proposes another way besides the above-mentioned these ways.
Hereafter, a forming process for a photoresist pattern disclosed in the above Patent Publication is described with reference to FIGS. 3(a) to 3(e).
First, a chemically amplified resist is applied on a semiconductor substrate 31 by means of the spin coating method to form a chemically amplified resist film 33 on the semiconductor substrate 31 (FIG. 3(a)). Next, a protective film 35 made of an organic material (e.g., polyethylene or an acidic polymer) is made on the chemically amplified resist film 33. This protective film 35 plays a role of protecting a catalyst (for example, an acid) from any factor, which will deactivate the catalyst, existing in the air keeping the exposed sample, the catalyst being generated at the exposed part of the chemically amplified resist film 33 on the case of exposing the same (FIG. 3(b)). Then, the chemically amplified resist film 33 is selectively exposed through the protective film 35 (FIG. 3(c)). After peeling the protective film 35, the substrate 31 is subjected to a bake treatment after exposure (FIG. 3(d)). After that, a photoresist pattern is formed by developing (FIG. 3(e)). Also, this Patent Publication has proposed a method for forming a protective film on a resist after exposure.
However, a throughput which a developing apparatus can develop is limited. That is, an about one hour delay is essential until a final treatment of lots. During this delay time, the acid is deactivated. The chemical filter is especially effective in reducing a concentration of ammonia inside the apparatus but not enough effective in dehydrating. Further, maintenance such as the filter exchange is timely necessary. On the other hand, the overcoat of the strong acid aqueous solution is excellent in its surroundings. However, it has a fateful drawback such that the acid to be used is so strong as pH2 to 3 that the apparatus, especially metal inside the applying apparatus is apt to be corroded by the acid.
A method disclosed in Japanese Unexamined Patent Publication No. Hei 7(1995)-295228 uses the protective film 35 made of polyethylene in order to protect deactivation of the acid, so that another process of removing the protective film is necessary. This is because a solvent used for removing the protective film 35 is a solvent of an aromatic compound such as benzene and different form a developer, such as an alkaline aqueous solution, to be used in a usual resist process.
On the other hand, by using the developer of the aromatic compound such as benzene, the protective film 35 can be removed at the same time of developing. However, by this method, usage of the developer increases and then, there arises a problem such as a bad influence to the environment. In the case of using an acidic polymer as a protective film, there is the same problem as in the case of using the strong acid aqueous solution, such that metal inside the applying apparatus is apt to be corroded by the acid.